Recently, computerization of information and increases in capacity of web content have advanced with extensive use of Internet and a rapid increase in communication speeds. Accordingly, electronic information storage demands have now increased throughout the world. To enable recording of large amounts of data, use of magnetic recording/reading devices, such as a hard disk drive (HDD), has increased rapidly along with demands for increased recording density, and a magnetic head or a magnetic medium which enables high recording density. In recent years, a magnetoresistive effect head has been mounted as a reproduction element in a magnetic recording/reading device, and a structure called spin valve has been used for the head, which utilizes a magnetoresistive effect of a multilayer film formed by stacking a ferromagnetic metal layer and a nonmagnetic layer. The magnetoresistive effect refers to a phenomenon where electric resistance is changed depending on an angle formed by magnetizations of two ferromagnetic layers with a nonmagnetic intermediate layer between them. The spin valve using the magnetoresistive effect has a structure of antiferromagnetic layer/ferromagnetic layer/nonmagnetic intermediate layer/ferromagnetic layer, and produces output by allowing magnetization of the ferromagnetic layer contacted to the antiferromagnetic layer to be substantially pinned by an exchange coupling magnetic field generated at an interface between the antiferromagnetic layer and the ferromagnetic layer, and magnetization of another ferromagnetic layer to be freely rotated by an external magnetic field. The ferromagnetic layer of which the magnetization is substantially pinned by the antiferromagnetic layer is called the pinning layer, and the ferromagnetic layer of which the magnetization is rotated by an external magnetic field is called the free layer.
In the past, a CIP (Current-In-Plane)-GMR (Giant Magnetoresistive) head has been used for the spin valve utilizing the magnetoresistive effect, which is used by flowing current in an in-plane direction of a stacked film. Recently, heads have been increasingly utilizing Tunneling Magnetoresistive (TMR) heads or CPP (Current-Perpendicular-to-Plane)-GMR heads, which are used by flowing current in a thickness direction of a stacked film.
There are two major reasons why the CIP-GMR head has been increasingly replaced with the TMR head or the CPP-GMR head in current head designs. First, since the TMR head or the CPP-GMR head can have increased reading output compared with the CIP-GMR head, a high SNR (output/noise ratio) may be achieved therein. Second, the CPP method in which current is flowed in a perpendicular direction of a stacked film is advantageous in improving linear recording density compared with the CIP method in which current is flowed in an in-plane direction of the stacked film. The linear recording density means bit density in a circumferential direction of a magnetic recording medium. Bit density in a radial direction of the medium is called track density. Both bit densities may be increased, thereby areal density of a magnetic recording/reading device is improved. Accordingly, reading resolution must be improved to increase linear recording density as an increased bit density is not helpful if a reader cannot read the bits. The reading resolution is an index showing what level of reading output can be kept during high density recording compared with during low density recording.
Currently, a magnetoresistive effect head is configured such that a magnetoresistive effect film is interposed between a lower magnetic shield and an upper magnetic shield, and reading resolution in a linear recording density direction greatly depends on an interval between the upper and lower magnetic shields. That is, as the interval between the upper and lower magnetic shields is reduced, the resolution in the linear recording density direction is increased, and high areal density may be more easily achieved. In the previous CIP-GMR head, the magnetoresistive effect film was electrically isolated from each of the upper and lower magnetic shields. Therefore, an insulating film was inserted between the magnetoresistive effect film and each of the upper and lower magnetic shields, and the interval between the upper and lower magnetic shields was difficult to reduce. On the other hand, a TMR head or a CPP-GMR head, in which current is flowed in a perpendicular direction of a stacked film, has an advantage in that the interval between the upper and lower magnetic shields may be reduced because the insulating film needs not be inserted between the magnetoresistive effect film and each of the upper and lower magnetic shields. In this way, the magnetoresistive effect head may be shifted from the CIP-GMR head to the TMR head or the CPP-GMR head with a goal of achieving higher output and improved reading resolution.
However, current CPP magnetoresistive effect films are generally extremely difficult to maintain thicknesses of less than about 25 nm because the film has a multilayer structure, and each layer has a minimum thickness necessary for keeping a magnetic property of the layer. Therefore, improvement in reading resolution may reach a limit in the near future based on the layer's minimum thickness. Accordingly, the interval between the upper and lower magnetic shields cannot be reduced to less than about 25 nm in a read head having the existing structure, which is a significant obstruction to achieving high areal density.
Thus, a differential read head has been proposed for improving resolution in the linear recording density direction. In the in-plane magnetic recording method, a signal magnetic field is generated from a recording bit only in a magnetization reversal region in recording bits written into a magnetic recording medium. On the other hand, in the perpendicular magnetic recording method, a signal magnetic field is always generated from each recording bit. Therefore, the perpendicular magnetic recording method is convenient for using the differential read head.
Japanese Patent Office (JPO) Pub. No. JP-A-2002-183915 discloses a read head structure of a magnetic recording/reading device using the perpendicular magnetic recording method, in which a pair of magnetoresistive effect films are connected in series via a conductive layer for differential operation. In the pair of magnetoresistive effect films, two free layers to be magnetosensitive portions to a signal magnetic field are disposed to be adjacently opposed to each other via a conductive layer. In addition, the pair of magnetoresistive effect films are set to have resistance change characteristics having reversed polarities in response to a magnetic field in the same direction. Thus, differential operation is enabled. In this case, resolution in the linear recording density direction is greatly affected by an inside distance between the free layers rather than an interval between the upper and lower magnetic shields. Therefore, even if the interval cannot be reduced, a thickness of the conductive layer inserted between the pair of magnetoresistive effect films may be decreased, thereby allowing for a high resolution in the linear recording density direction.
Furthermore, JPO Pub. No. JP-A-2003-69109 discloses a more detailed structure of a differential read head, in which two free layers may show resistance change characteristics having reversed polarities in response to a magnetic field in the same direction.
Also, JPO Pub. No. JP-A-2008-85219 discloses a structure of a differential read head, in which one of the pair of magnetoresistive effect films shows negative magnetic resistance. In addition, it describes a configuration where a free layer is thinner, thereby use efficiency of a magnetoresistive effect film on a low output side is improved so that outputs of the films are effectively the same. However, if the free layer is thinner, since unidirectional anisotropy in the free layer due to magnetic domain control is also enhanced, an effect of improving sensitivity to a medium magnetic field is obtained, but two correlative parameters of a free layer and magnetic domain control need to be controlled, leading to difficulty in designing a head which works properly.
JPO Pub. No. JP-A-2004-227749 discloses a structure of a read head, in which high resolution is achieved even if the upper and lower magnetic shields are not provided.
However, none of these references disclose heads which can achieve the high areal density sought by current recording/reading applications, and therefore, a read head which is capable of achieving improved reading resolution which avoids the problems described previously would be greatly beneficial to increasing the areal density possible with magnetic recording/reading heads.